Various types of monitoring systems to supervise the composition of exhaust gases in internal combustion engines have previously been proposed; such circuits, generally, provide an output signal which indicates whether the internal combustion engine is operating with a fuel and air supply which is either lean, that is, with a fuel-air ratio of greater than stoichiometric (.lambda.&gt;1), or rich (.lambda.&lt;1). This output signal can also be utilized to control the relative proportion of fuel and air being supplied to the internal combustion engine so that it will have the desired proportion and come as close to stoichiometric levels as possible. Reference is made to U.S. Pat. Nos. 4,021,326 and 3,841,987, Friese et al, for example, for a sensor of this type; and to U.S. Pat. Nos. 3,782,347, Schmidt et al. and 3,759,232, Wahl et al. all assigned to the assignee of the present application, for control systems suitable for use therewith. These, and other control and monitoring systems have the advantage that they rapidly react to changes in the composition of the exhaust gases. The .lambda.-sensors which are based on oxygen ion concentration bodies with solid electrolytes react rapidly to changes in the composition of the exhaust gases. Their output signal changes essentially instantaneously between discrete values upon change in the composition of the exhaust gases between reducing and oxidizing state. When the concentration of oxygen drops or, respectively, the partial pressure of the oxygen drops to approximately zero, a characteristic voltage jump is obtained which can be readily evaluated in an electronic control circuit; see, for example, the aforementioned U.S. Pat. No. 3,759,232. The .lambda.-sensors, however, are not suited to provide a signal which is readily evaluated and representative of oxygen partial pressure. Basically, this type of sensor is primarily useful to prevent operation of the engine with a mixture which is too rich, that is, at a .lambda. of less than unity. If the operating conditions of the engine are so set, or the design of the engine is such that it should be operated with a mixture which is relatively lean, that is, with a mixture in which there is an excess of oxygen present with respect to the stoichiometric proportion, then the exact value of the excess oxygen cannot be sensed by the type of .lambda.-sensor employed in the aforementioned control systems and a lean fuel-air mixture cannot be controlled by such a sensor by controlling the relative proportion of fuel and air being supplied to the engine.